These are really simple and easy to sew up. I very casually only sorta measure a package when I have to make one, but since they're used for years, at this point I'm not making a ton. I sew the side seam and follow up with zigzag stitch. Then I sew the bottom. Then I pin and hem the top. People aren't looking too clearly at the stitching or hems so I either pin or iron but never both - it takes five minutes to make one of these things.
Then I either tie the top closed (bag-style) or I fold the top inside the package like a pillowcase:
Finish with a ribbon and you're done!
This is a great "thread user" project, use up the end of a spool that you don't need around, since long zigzag stitching uses lots of thread and as I said, these are cheap and casual and nobody pays attention to whether your thread matches. They'll get re-used for years, unlike paper gift bags. They can hold a ton of weight, get thrown in the back of a car and they don't get mashed up.
Dark fabric with busy patterns works best because it's gift wrap, you can sometimes see through white!
In my family we set a price limit every year and every person gets gifts from everybody so the pile of wrapping paper used to be bags and bags of throw away paper, but between my sister introducing random yards of fabric and me making them into bags we keep and send around, we're barely trashing or recycling anything these days! Less mess and less time spent on wrapping, we are definitely going the right direction!
